Keep max-age within an expiring image URL's lifetime (closes #63)

Both image routes sent Cache-Control: public, max-age=31536000,
immutable, so a browser or proxy could keep serving an image for a year
after its signed or encrypted URL had expired. The header is now built
from the request's Expires: max-age is the whole seconds left until the
URL expires, never negative, or one year for a URL with no expiry.
ToImageRequest now carries an encrypted URL's expiry onto the request,
as the image route already does with exp. immutable stays: it only stops
revalidation while a copy is fresh, and freshness now ends at the
expiry. README.md documents the header.

Model: opus-5-5

internal/encurl/encurl.go

@@ -103,7 +103,8 @@ func (g *Generator) Parse(token string) (*Payload, error) {
 }
 
 // ToImageRequest converts the payload to an ImageRequest.
-// Applies default values for omitted optional fields.
+// Applies default values for omitted optional fields. An ExpiresAt of 0, a URL
+// that never expires, gives the zero Expires.
 func (p *Payload) ToImageRequest() *imgcache.ImageRequest {
 	format := p.Format
 	if format == "" {
@@ -120,6 +121,11 @@ func (p *Payload) ToImageRequest() *imgcache.ImageRequest {
 		fitMode = DefaultFitMode
 	}
 
+	var expires time.Time
+	if p.ExpiresAt != 0 {
+		expires = time.Unix(p.ExpiresAt, 0)
+	}
+
 	return &imgcache.ImageRequest{
 		SourceHost:  p.SourceHost,
 		SourcePath:  p.SourcePath,
@@ -131,6 +137,7 @@ func (p *Payload) ToImageRequest() *imgcache.ImageRequest {
 		Format:  format,
 		Quality: quality,
 		FitMode: fitMode,
+		Expires: expires,
 	}
 }